Linepipe Steel With Alternative Carbon Steel Compositions For Enhanced Sulfide Stress Cracking Resistance

ABSTRACT

The present disclosure relates to methods and compositions of linepipe steels that transport one or both of crude oil and natural gas. More particularly, the present disclosure relates sulfide stress cracking resistance of carbon steels for use as linepipe in transporting crude oil and natural gas by alternative carbon steel compositions comprising relatively low-manganese contents and/or low-carbon contents, alone or in combination with hydrogen trapping precipitates.

FIELD OF THE INVENTION

This application relates to methods and treatments of linepipe steels that transport crude oil and natural gas.

BACKGROUND OF THE INVENTION

This application relates to linepipe steels that transport crude oil and/or natural gas and, more particularly, to linepipe steels comprising alternative carbon steel compositions and methods and treatments thereof for enhanced sulfide stress cracking resistance.

High-strength metallic materials, such as high-strength steels, are commonly used, cost-effective materials for forming linepipe for transporting crude oil and natural gas mined from oil or gas fields. These high-strength, cost-effective materials may be selected to permit transportation of large volumes of crude oil or natural gas without compromising the integrity of the linepipe. That is, the high-strength nature of the materials may permit transportation at relatively higher pressures, or the use of relatively thinner linepipe wall thickness having relatively larger diameters to increase flow volume, and the like. As such, significant cost reduction may be realized from use of such high-strength materials in terms of, for example, material and construction costs.

However, crude oil and natural gas may comprise, among other components, hydrogen sulfide (H₂S) (which may be in relatively high or low concentrations, e.g., extreme to mild sour conditions), as well as other components, such as water, carbon dioxide, chloride, and the like. The presence of H₂S can detrimentally interfere with the operability of a steel linepipe due to sulfide stress cracking (SSC). SSC is a form of hydrogen embrittlement induced by atomic hydrogen that is produced by sour corrosion due to the presence of H₂S on a metal's (e.g., steel's) surface. The products of the sour corrosion include atomic hydrogen, which may be absorbed by the linepipe material, resulting in subsequent SSC. Moreover, the H₂S in crude oil and natural gas can further act as an SSC catalyst because the sulfur in the H₂S prevents recombination of atomic hydrogen into H₂, thereby maintaining a high concentration of atomic hydrogen available for absorption by the linepipe and, thus, formation of SSC.

SSC susceptibility of linepipe carbon steels may be dependent on a number of factors such as, for example, the microstructure of the compositional metal, inclusion and precipitate distribution, chemical composition, and the like, and combinations thereof. High-strength metallic materials, such as high-strength carbon steels and/or weldments, having heat affected zones with high hardness (e.g., a hardness greater than 248 Vickers hardness number) commonly used to form linepipe may be particularly susceptible to SSC.

SSC can cause linepipe to lose ductility (e.g., elongation to rupture) and increase cracking susceptibility, thereby causing the linepipe to fail at stresses below its nominal yield strength when subjected to the mechanical stresses of transporting crude oil and/or natural gas (e.g., tensile stresses, residual or applied, and the like). Structural failure of formed parts can result in severely hazardous environmental and operational conditions, as well as substantial economic losses. Accordingly, metallic materials for use as linepipe for transporting crude oil and natural gas comprising H₂S require high resistance to sour corrosion and/or high resistance to SSC per industry standard requirements.

SUMMARY OF THE INVENTION

This application relates to structural carbon steels that are used to form linepipe that transport crude oil and/or natural gas and, more particularly, to linepipe carbon steels comprising alternative steel compositions and methods and treatments thereof for enhanced sulfide stress cracking resistance.

In one or more aspects, the present disclosure provides a carbon steel composition comprising: manganese in an amount of equal to less than about 1.6% by weight or less than about 1.3% by weight of the carbon steel composition.

In one or more aspects, the present disclosure provides a carbon steel composition comprising: carbon in an amount of equal to or less than about 0.025% by weight of the carbon steel composition.

BRIEF DESCRIPTION OF THE DRAWINGS

The following figures are included to illustrate certain aspects of the disclosure, and should not be viewed as exclusive configurations. The subject matter disclosed is capable of considerable modifications, alterations, combinations, and equivalents in form and function, as will occur to those skilled in the art and having the benefit of this disclosure.

FIG. 1 illustrates comparative TMCP methods demonstrating the differences between a traditional low temperature TMCP (tTMCP) methodology and the higher temperature TMCP (htTMCP) methodology of the present disclosure.

FIGS. 2A-2D show representative reconstructed austenite structures at Finishing Rolling Temperature (FRT) and resultant micrographs of carbon steel formed according to the ltTMCP and the htTMCP of FIG. 1 .

FIGS. 3A-3C show representative micrographs of conventional carbon steels formed according to either ltTMCP or the htTMCP of the present disclosure; FIGS. 3D and 3E show representative micrographs of low-manganese carbon steels comprising vanadium formed according to the htTMCP of the present disclosure.

FIGS. 4A and 4B show representative micrographs of low-carbon carbon steels comprising vanadium formed according to the htTMCP of the present disclosure.

FIG. 5A shows a representative micrograph of a low-manganese carbon steel comprising niobium and vanadium of the present disclosure; FIGS. 5B-5D show representative micrographs of combination low-manganese and low-carbon carbon steels comprising niobium and/or vanadium of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

This application relates to linepipe steels that transport crude oil and/or natural gas and, more particularly, to linepipe steels comprising alternative steel compositions and methods and treatments thereof for enhanced sulfide stress cracking resistance.

As used herein, the term “linepipe,” and grammatical variants thereof, refers to any tubular linepipe steel that transports crude oil and/or natural gas (or other liquids and/or gases), including pipelines, flowlines, risers, any other transport conduit, and the like, without limitation. When one or more aspects of the present disclosure is described with reference to “pipeline,” for example, the disclosure is equally applicable to any other type of linepipe, without limitation, unless specified otherwise.

The present disclosure advantageously enhances the sulfide stress cracking (SSC) resistance of carbon steels for use as linepipe in transporting crude oil and natural gas by alternative steel compositions comprising one or both of relatively low-manganese contents and/or low-carbon contents, either of which may be alone or in combination with (1) hydrogen trapping precipitates (e.g., niobium, vanadium, and the like) and/or (2) one or more transition metal elements (e.g., nickel, chromium, molybdenum, and the like) and/or (3) the chemical element boron. That is, the present disclosure provides for alternative steel compositions having relatively low-manganese contents alone or in combination with any one or all of the aforementioned (1), (2), and (3); relatively low-carbon contents alone or in combination with any one or all of the aforementioned (1), (2), and (3); and both relatively low-manganese contents and low-carbon contents alone or in combination with any one or all of the aforementioned (1), (2), and (3). As used herein, the term “hydrogen trapping precipitates,” and grammatical variants thereof, refers to solid precipitates precipitated during manufacturing of a formed carbon steel, which are capable of trapping and reducing hydrogen diffusion.

The present disclosure further advantageously enhances the SSC resistance of carbon steels having the alternative compositions described herein by providing one or more alternative thermo-mechanically controlled processes (TMCP) and/or one or both of two post-steel plate or pipe (e.g., welded or seamless) production heat treatment processes (termed either (1) “interim austenitization treatment processes” or (2) “final heat treatment processes,” as discussed hereinbelow, and grammatical variants thereof). It is to be understood, however, that while the alternative carbon steel compositions are described primarily with reference to these alternative processes provided hereinbelow, the alternative carbon steel compositions of the present disclosure are equally applicable to other manufacturing processes, without limitation, such as traditional TMCP, Electric Resistance Welded (ERW) Pipe, Spiral Welded Pipe, Seamless Pipe, and the like, and any combination thereof.

Alternative Carbon Steel Compositions

Traditional metallic metal compositions (e.g., carbon steels) for use in forming linepipe typically have one or both of a manganese content of greater than 1.4 wt. % and a carbon content of maximum 0.16 wt. %, among other elemental components. These traditional manganese and carbon contents are known to enhance certain mechanical properties, such as increasing depth of hardening and improving strength and toughness of the carbon steel.

Different than traditional compositions, the present disclosure provides alternative carbon steel compositions, which may be used to produce formed carbon steel (e.g., plates, seamless pipes, and the like) using, for example, TMCP as described herein, among other alternative steel manufacturing processes. The alternative carbon steel compositions described herein advantageously enhance the SSC resistance of the produced linepipe without compromising desired mechanical properties.

More specifically, the present disclosure provides carbon steel compositions for use as linepipe that have lower manganese (Mn) content and/or lower carbon (C) content, alone or in combination with (1) hydrogen trapping precipitates and/or (2) one or more transition metal elements and/or (3) the chemical element boron (B), as compared to traditional carbon steel compositions. The alternative carbon steel compositions described herein further meet industry requirements for the chemical composition of carbon steels used as linepipe (e.g., API 5L, EN ISO 3183, CSA Z245.1, and the like), including minimum yield strength of greater than 52 ksi.

Without being bound by theory, it is believed that the lower manganese and/or lower carbon content carbon steel compositions described herein enhance SSC resistance by reducing microstructural constituents, such as martensite/austenite constituents (M/A constituents), among other potential constituents, believed to be detrimental to SSC resistance. The alternative carbon steel compositions of the present disclosure comprising reduced manganese and/or reduced carbon can minimize or otherwise eliminate the formation of these hard phases. In one or more aspects, as described above, the alternative lower manganese and/or lower carbon content carbon steel compositions may further comprise one or more transition metals (e.g., nickel, niobium, vanadium, niobium+vanadium, chromium+molybdenum, and the like) and/or boron. Without being bound by theory, for example, it is believed that the inclusion of nickel in low-manganese content compositions may increase hardenability to promote the formation of lath bainite, and may further increase the intrinsic toughness of the formed carbon steel (although nickel may be included in the alternative low-carbon content carbon steels described herein, as well). Similarly, the inclusion of Cr and/or Mo in low manganese content compositions may increase hardenability to promote the formation of lath bainite, and may produce the precipitates of Cr/Mo carbides potentially resulting in the increase of yield strength and/or acting as hydrogen trapping sites. Without being bound by theory, for example, it is believed that the inclusion of boron in low-manganese content compositions may increase hardenability to promote the formation of lath bainite. In one or more aspects, as described above, the alternative lower manganese content carbon steel compositions may further have the carbon range of about 0.01 wt. % to about 0.15 wt. % (e.g., about 0.03 wt. % to about 0.12 wt. %), encompassing any value and subset therebetween, to provide higher strength (YS 52100 ksi) by promoting lath bainite, such as having equal to or greater than about 50 vol. % lath bainite, including up to 100 vol. %, encompassing any value and subset therebetween. In some instances, the alternative carbon steel compositions comprising the alternative low-carbon content of the present disclosure can be used to produce formed carbon steels that are substantially hard phase free, such as having equal to or greater than about 90 vol. % ferrite, including up to 100 vol. % ferrite, encompassing any value and subset therebetween, depending on the particular manufacturing process selected (e.g., traditional v. the alternative TMCP process and/or additional heat treatment process(es)).

In various aspects of the present disclosure, the amount of manganese included in a carbon steel composition for use as linepipe may be in the range of equal to or less than about 1.6 wt. % of the carbon steel composition in total, such as about 0.6 wt. % to about 1.6 wt. % or about wt. % to about 1.3 wt. %, or about 0.8 wt. % to about 1.2 wt. %, encompassing any value and subset therebetween.

In various aspects of the present disclosure, the amount of carbon included in a carbon steel composition for use as linepipe may be in the range of equal to or less than about 0.025 wt. % of the carbon steel composition in total, such as about 0.01 wt. % to about 0.025 wt. %, or about wt. % to about 0.02 wt. %, or about 0.01 wt. % to about 0.015 wt. %, encompassing any value and subset therebetween.

Accordingly, in one or more aspects of the present disclosure, an alternative carbon steel composition may have a manganese content in the range of about 0.6 wt. % to about 1.6 wt. % or about 0.6 wt. % to about 1.3 wt. % of the carbon steel composition, in combination with a carbon content in the range of about 0.01 wt. % to about 0.025 wt. % of the carbon steel composition, encompassing any value and subset therebetween.

It has been presently observed, as provided in the examples hereinbelow, that even minimal reduction of manganese and/or carbon compared to traditional compositions (e.g., reduction by 0.005, or 0.01, or 0.02 wt. %) can have significant impact on SSC resistance. However, in some instances, depending on the remaining elemental composition of the carbon steel, significantly decreased manganese and/or carbon may result in a reduction of yield strength beyond typical standard industry requirements for use as linepipe. In one or all aspects, these yield strength reductions may be overcome using the high temperature TMCP and/or interim austenitization and/or final heat treatment methodologies provided in the present disclosure.

In one or all aspects, alone or in combination with the alternative TMCP treatment described herein, to further enhance the yield strength of formed carbon steel using the alternative carbon steel compositions of the present disclosure, one or more additional elements may be included. For example, a carbon range of about 0.03 wt. % to about 0.15 wt. %, encompassing any value and subset therebetween, may increase hardness and/or strength of the alternative low manganese content carbon steels described herein.

In one or all aspects, alone or in combination with the alternative TMCP treatment described herein, to further enhance the yield strength of formed carbon steel using the alternative carbon steel compositions of the present disclosure, one or more additional elements may be included. For example, the inclusion of nickel may increase hardness and/or toughness of the alternative low manganese and/or low carbon content carbon steels described herein. In one or more aspects, the alternative carbon steel compositions may have a nickel content in the range of about 0.15 wt. % to about 1.0 wt. % of the carbon steel composition in total, encompassing any value and subset therebetween. The inclusion of nickel may be particularly preferred for inclusion in the low-manganese content carbon steel compositions described herein.

In one or all aspects, additional boron may be included to increase hardenability, and accordingly promotes the formation of lath bainite, in the process per the present disclosure of the alternative carbon steel compositions.

In one or all aspects, the one or more additional elements may be included to form hydrogen trapping precipitates upon final heat treatment per the present disclosure. These hydrogen trapping precipitates are typically produced by precipitates of one or both of niobium or vanadium within the alternative carbon steel compositions of the present disclosure. Not only do the precipitates provide increased yield strength to compensate for any softening by a matrix change due to lean composition, they also provide the trapping sites for hydrogen and contribute to SSC resistance. The hydrogen trapping precipitates are in the form of, for example, niobium carbide (NbC) and/or vanadium carbide (VC) or mixtures thereof. In one or more aspects of the present disclosure, when nitrogen is included in the alternative carbon steel compositions of the present disclosure, the hydrogen trapping precipitates may be in the form of niobium carbonitride (NbCN) or vanadium carbonitride (VCN).

In one or more aspects of the present disclosure, niobium, when alone, may be present in the alternative carbon steel compositions of the present disclosure in an amount of about 0.02 wt. % to about 0.10 wt. % of the carbon steel composition, such as about 0.02 wt. % to about 0.04 wt. %, or about 0.04 wt. % to about 0.08 wt. %, or about 0.05 wt. % to about 0.10 wt. %, encompassing any value and subset therebetween. Similarly, vanadium, when alone, may be present in the alternative carbon steel compositions of the present disclosure in an amount of about wt. % to about 0.10 wt. % of the carbon steel composition, such as about 0.02 wt. % to about wt. %, or about 0.04 wt. % to about 0.08 wt. %, or about 0.05 wt. % to about 0.10 wt. %, encompassing any value and subset therebetween. When both niobium and vanadium are present, in combination they may be present in the alternative carbon steel compositions of the present disclosure in an amount of about 0.02 wt. % to about 0.15 wt. % wt. % of the carbon steel composition in total, such as about 0.02 wt. % to about 0.04 wt. %, or about 0.04 wt. % to about 0.08 wt. %, or about 0.05 wt. % to about 0.10 wt. %, encompassing any value and subset therebetween.

The formation of fine grained and well distributed precipitates capable of trapping hydrogen and thus reducing or eliminating diffusible hydrogen can contribute to enhanced SSC resistance, particularly if the manganese and/or carbon content has significantly reduced yield strength. In some aspects, the hydrogen trapping precipitates may be micro- or nano-sized, such as having a unit size of about 1 nm to about 20 nm, encompassing any value and subset therebetween. As used herein, the term “unit size,” and grammatical variants thereof, refers to the size of an object (e.g., a precipitate having spherical or otherwise irregular shaping) that is capable of passing through a particular square area.

Other elements (in addition to one or more of manganese (Mn), carbon (C), nickel (Ni), niobium (Nb), and vanadium (V)) that may be included in the alternative carbon steel compositions of the present disclosure include, but not limited to, phosphorous (P), sulfur (S), silicon (Si), aluminum (Al), chromium (Cr), molybdenum (Mo), titanium (Ti), nitrogen (N), calcium (Ca), boron (B), and any combination thereof. These other elements may be present in order to further increase desired properties of the formed carbon steel from the alternative lower manganese and/or lower carbon content carbon steel compositions described herein.

The selection, combination, and amount of any additional elements (including nickel, niobium, and vanadium) may be based on a number of factors including, but not limited to, the particular manufacturing process selected to produce the formed steel, the desired qualities of the produced formed steel, the particular use of the subsequent linepipe, and the like, and any combination thereof.

In one or more aspects of the present disclosure, phosphorous may be present in the alternative carbon steel compositions in an amount of less than or equal to about 0.015 wt. %, such as in the range of about 0.0005 wt. % to about 0.015 wt. %; sulfur may be present in an amount of less than or equal to about 0.025 wt. %, such as in the range of about 0.0005 wt. % to about 0.025%; silicon may be present in an amount of equal to or less than 0.45 wt. %, such as in the range of about 0.01 wt. % to about 0.45 wt. %; aluminum may be present in an amount of equal to or less than about 0.1 wt. %, such as in the range of about 0.01 wt. % to about 0.1 wt. %; chromium may be present in an amount in the range of about 0.1 wt. % to about 0.75 wt. %, or about 0.1 wt. % to about wt. %; molybdenum may be present in an amount in the range of about 0.1 wt. % to about 0.5 wt. %; titanium may be present in an amount in the range of about 0.005 wt. % to about 0.1 wt. %; nitrogen may be present in an amount of greater than or equal to about 0.01 wt. %, such as in the range of about 0.001 wt. % to about 0.01 wt. %; calcium may be present in an amount of greater than or equal to about 0.005 wt. %, such as in the range of about 0.0001 wt. % to about 0.005 wt. %; and boron may be present in an amount of about 0.0005 wt. % to about 0.003 wt. %, each by wt. % of the carbon steel composition in total, encompassing any value and subset therebetween.

In one or more aspects, for example, the alternative low manganese and/or low carbon content carbon steel compositions of the present disclosure may include one or more of nickel, vanadium, and/or niobium. In one or more aspects, the alternative low manganese and/or low carbon content carbon steel compositions of the present disclosure may include one or more of boron, chromium, and/or molybdenum. In some aspects, the inclusion of a particular element may be more advantageous when the resultant formed carbon steel is a seamless pipe, such as nickel, niobium, vanadium, boron, chromium, and/or molybdenum. In some aspects the inclusion of a particular element may be more advantageous when the resultant formed carbon steel is a non-seamless pipe (e.g., a plate formed into a tubular linepipe, such as by welding), such as nickel, niobium, and/or vanadium.

It is to be understood that regardless of the manufacturing process for forming the carbon steel (e.g., traditional TMCP v. alternative) or the type of formed carbon steel (e.g., non-seamless v. seamless), any of the additional elements described herein and in any of the concentrations described herein are equally applicable alone or in combination for forming the alternative low manganese and/or low carbon content steels of the present disclosure.

Alternative Carbon Steel Manufacturing Processes

The alternative steel compositions described above may be prepared and formed into linepipe using any traditional process, without limitation, as described above. Alternatively, the alternative carbon steel compositions may be prepared and formed into linepipe using or one or more alternative processes described hereinbelow.

Carbon steels for use in linepipe are traditionally formed using a thermo-mechanically controlled process (TMCP) under specific conditions. TMCP is a metallic rolling technique in which the mechanical properties of the metal are controlled using a hot deformation process in a rolling mill and transformed during a controlled accelerated cooling process. TMCP is a hot deformation process, and such terminology may be used interchangeably in the industry. Traditional TMCP utilizes relatively low reheating temperature (RHT) (e.g., equal to or less than or equal to 1150° C.) and/or relatively low finishing rolling temperature (FRT) (e.g., less than or equal to 900° C.), including one or more deformation steps at these relatively low temperatures, such as hot rolling, along with relatively short interpass times (i.e., time between two consecutive deformation steps) to minimize austenite grain growth. These lower RHT/FRT temperatures are traditionally known to enhance the mechanical properties of the metal by promoting the formation of fine grain sized microstructure transformed from finer prior austenite grains. As a tradeoff for achieving desired mechanical properties, heavy, and costly, rolling equipment is generally required to achieve deformation of the metal at these low temperatures.

Different than traditional methods, the present disclosure provides alternative TMCP manufacturing methodologies that advantageously enhance the SSC resistance of the produced linepipe without compromising other desired mechanical properties. The TMCP methodologies of the present disclosure may be used alone or in combination with the additional subsequent heat treating methodologies.

The present disclosure provides a TMCP process that employs relatively higher RHT and/or higher FRT temperatures compared to traditional TMCP, including one or more deformation (e.g., hot rolling) steps at relatively higher temperatures. It is to be appreciated that small variations in RHT and FRT can have significant effect on microstructure, strength, and SSC resistance of a carbon steel. For these reasons, traditional low temperature RHT (<1150° C.) and low temperature FRT (<900° C.) have been used to minimize austenite grain growth. The present disclosure establishes that higher RHT and higher FRT, even if by several degrees, can advantageously alter the microstructure of the resultant steel and its SSC resistance, as described hereinbelow.

Additionally, in some instances, the TMCP described herein further employs one or more relatively longer extended interpass times, as well, to promote recovery and or recrystallization of deformed austenite; extended interpass time parameters are described hereinbelow and, unlike traditional interpass practice, the extended interpass times described herein are controlled such that the interpass time is managed and not otherwise random. After reaching FRT, the TMCP methods include quenching or cooling at a cooling rate. Quenching includes accelerated cooling, wherein a fluid is used to increase cooling rate (as opposed to air cooling); other quenching methods may also be employed, without departing from the scope of the present disclosure. Accelerated cooling may enhance various performance properties of the TMCP-produced steel, such as enhanced low temperature toughness and high fracture toughness. After quenching, a formed carbon steel is produced.

As used herein, the term “formed carbon steel” or “formed steel,” and grammatical variants thereof, refers to a carbon steel composition (i.e., the alternative carbon steel compositions of the present disclosure) that has been formed into the shape of a plate (also referred to as a sheet) or seamless pipe and quenched (e.g., cooling by any means, such as to room temperature or other appropriate temperature), such as by the TMCP methods described herein. The formed steel is suitable for use as linepipe for use as pipeline, as described herein, and meet required industry standards (e.g., API 5L, EN ISO 3183, CSA Z245.1, and the like).

Using the alternative TMCP methods of the present disclosure result in coarser (or relatively coarser compared to traditional processes) austenite grain sizes that promote the formation of lath (i.e., long and slender) bainite. The coarser austenite grain sizes enhance the hardenability of the formed steel and favor the formation of lath bainite over the formation of traditional, acicular ferrite and/or granular bainite. The resultant metal, and linepipe manufactured therefrom, benefits from the strong mechanical properties of the lath bainite (e.g., tensile properties, such as low temperature toughness, and the like) while exhibiting enhanced SSC resistance. It is to be understood, however, that although the methods of the present disclosure promote lath bainite, other microstructure, such as acicular ferrite and/or granular bainite, may also be present in the produced metal, without departing from the scope of the present disclosure. Alternatively, it is to be understood, as previously stated, that the alternative low manganese and/or low carbon content carbon steel compositions of the present disclosure may be formed using traditional manufacturing processes (e.g., traditional TMCP), which may produce carbon steels having equal to or greater than about 90 vol. % ferrite.

Various aspects of the present disclosure are described hereinbelow by providing comparative description and examples to traditional methodology. It is to be understood that the term “TMCP” will refer to the methodologies of the present disclosure and all “traditional” TMCP references will be labeled as such or similarly. Further, although various specific carbon steel compositions are described herein, it is to be appreciated that the TMCP methodologies of the present disclosure for enhancing SSC resistance are applicable to any and all steel compositions for use in manufacturing linepipe and meeting required industry standards (e.g., API 5L, EN ISO 3183, CSA Z245.1, and the like), without limitation.

Referring first to FIG. 1 , illustrated are comparative TMCP methods graphically demonstrating the differences between a traditional low temperature TMCP (tTMCP) methodology and the higher temperature TMCP (htTMCP) methodology of the present disclosure. Representative, and non-limiting, deformation (e.g., hot rolling) steps are shown as starbursts, further illustrating extended non-limiting interpass times as part of the htTMCP methods of the present disclosure.

As comparatively shown, the traditional ltTMCP demonstrates lower slab reheating temperature (ltRHT) in order to minimize austenite grain growth (keeping the grain size small), and also demonstrates relatively lower finishing rolling temperature (ltFRT) in order to maximize hot deformation below the non-recrystallization temperature (Tnr) and achieve a pancaked or fine grain austenite structure. The ltTMCP produces a primarily fine grained microstructure composed of acicular ferrite, granular bainite or ferrite, or combinations thereof.

With continued reference to FIG. 1 , as comparatively shown, the htTMCP of the present disclosure demonstrates a higher slab reheating temperature (htRHT) to optimize austenite grain growth (encouraging large(r) austenite grain size), and also demonstrates relatively higher finishing rolling temperature (htFRT) in order to minimize hot deformation below the Tnr and prevent pancaking or deformation of the formed grain structures. The extended interpass time shown (one, non-limiting example shown) in the htTMCP plot of FIG. 1 may promote austenite recovery and recrystallization at the higher temperatures used (e.g., greater than about 1000° C.).

FIGS. 2A-2D show representative reconstructed austenite structures at FRT and resultant micrographs of formed steel according to the ltTMCP and the htTMCP of FIG. 1 . FIG. 2A and FIG. 2B show reconstructed austenite structure at ltFRT and htFRT, respectively of FIG. 1 . The reconstruction of austenite structure was completed through the use of typical orientation relationships between parent austenite phase and daughter bainitic phases based on Electron Back Scattered Diffraction (EBSD) techniques. As shown in FIG. 2B, the austenite structure according to the htTMCP of the present disclosure produces comparatively larger austenite grain structures. Indeed, the average prior austenite grain size (PAG) according to the traditional ltTMCP method was measured at 13.7 μm (FIG. 2A), whereas the average PAG according to the htTMCP method of the present disclosure was measured at 24.9 μm (FIG. 2B). Generally, the average PAG according to the htTMCP methods of the present disclosure are in the range of about 20 μm to about 50 μm, encompassing any value and subset therebetween, such as about 30 μm, or about 40 μm. FIGS. 2C and 2D show micrographs of the resultant formed steel after accelerated cooling according to the ltTMCP and the htTMCP, respectively, of FIG. 1 . As shown, the coarser austenite structure of the htTMCP method of the present disclosure promoted a lath bainite microstructure, as shown in FIG. 2D, which is absent or less apparent in the traditional ltTMCP microstructure shown in FIG. 2C comprising primarily fine(r) grained ferritic and granular bainitic microstructures.

In various aspects of the present disclosure, the RHT may be in the range of greater than or equal to about 1175° C. to about 1350° C., encompassing any subset and value therebetween, such as in the range of about 1200° C. to about 1300° C., more preferably between 1200° C. to 1250° C. The heating rate is not considered to be particularly limiting to reach the desired RHT; in some aspects, the heating rate to RHT may be in the range of about 0.1° C. per second (° C./sec) to about 100° C./sec, encompassing any value and subset therebetween. In one or more aspects, the RHT may be maintained stable for a predetermined length of time (e.g., depending on the steel composition and part size to be produced therefrom, depending on the RHT, and the like, and any combination thereof), such as greater than about one (1) minute to about ten (10) hours, encompassing any value and subset therebetween, such as greater than about 1 hour to about 5 hours. The time period in which the RHT is maintained is also referred to in the industry as the period of “austenitizing.” Accordingly, the austenitizing period described in the present disclosure is performed at a relatively higher temperature compared to traditional TMCP.

In various aspects of the TMCP methods described herein, the FRT may be in the range of the RHT to greater than about Ar3, encompassing any subset and value therebetween, such as about 910° C. to about 1000° C., encompassing any value and subset therebetween. At least one or more extended interpass durations may be in the range of about 10 seconds to about 10 minutes, such as about 30 seconds to about 10 minutes, encompassing any value and subset therebetween. The at least one or more extended interpass durations are performed at or above a temperature in the range of about Tnr (the non-recrystallization temperature) to about (Tnr+200° C.° C.) (i.e., the Tnr temperature plus 200° C.), such as in the range of about Tnr to about Tnr+100° C.), or in the range of about Tnr to about Tnr+50° C.), encompassing any value and subset therebetween. For example, in various aspects, at least one or more extended interpass durations are performed at or above a temperature of about 950° C., encompassing any subset and value therebetween, such as about 30 seconds to about 10 minute. In various aspects, one or more of the extended interpass durations may be in the temperature range of at least about 950° C. to about 1175° C., encompassing any value and subset therebetween.

Finally, the accelerated cooling (also referred to as quenching) for use in various aspects of the TMCP methods of the present disclosure may be in the range of about 1° C./sec to about 300° C./sec, encompassing any value and subset therebetween, such as about 5° C./sec to about 100° C./sec. The accelerated cooling may be to a temperature of equal to or less than about 500° C., such as in the range of about 100° C. to about 400° C., or about 20° C. (room temperature) to about 400° C., encompassing any value and subset therebetween.

In one or more aspects, the present disclosure incorporates a final heat treatment step after completion of a high temperature TMCP process (i.e., after cooling, and whether the cooled steel is formed or otherwise welded into a pipe). The high temperature TMCP and the final heat treatment step synergistically further enhance SSC resistance without compromising desired mechanical properties (e.g., tensile properties, low temperature toughness/resistance, hydrogen induced cracking, and the like) and with minimal manufacturing costs. It is to be understood that while the final heat treatment process of the present disclosure is described with reference to high temperature TMCP steel formation processes, it is equally applicable to any carbon steel formed by other methodologies, such as traditional TMCP, Seamless Pipe, Electric Resistance Welded (ERW) Pipe, Spiral Welded Pipe, and the like, and any combination thereof.

In one or more aspects, steel formed after completion of a high temperature TMCP process (or other formed steel process) may be subsequently heated (after accelerated cooling to a desired temperature) to a temperature in the range of about (Ac1−300° C.) (i.e., the Ac1 temperature less 300° C.) to about Ac1, such as in the range of about (Ac1−200° C.) to about Ac1, or in the range of about (Ac1−100° C.) to about Ac1, and preferably in the range of about (Ac1−300° C.) to less than Ac1, encompassing any value and subset therebetween. Ac1 is the lowest temperature at which austenite begins to form during heating at a specified rate and, accordingly, is not a static temperature. In one or more aspects, the heating rate may be in the range of about per second (° C./sec) to about 100° C./sec, encompassing any value and subset therebetween; such rates may depend on a number of factors including, but not limited to, the type of heating used (e.g., induction heating v. non-induction heating). In various aspects, the heating temperature for the final heating step after high temperature TMCP may be preferably relatively close to but less than the Ac1 temperature to maximize SSC resistance. Heat treatment above Ac1 may potentially cause the formation of hard phase from reverted austenite during cooling, which may deteriorate SSC resistance.

Upon reaching the desired temperature, it is maintained stable for a predetermined length of time, such as in the range of greater than about one (1) minute to about 10 hours, encompassing any value and subset there between, such as about 10 minutes to about 8 hours, or about 10 minutes to about 6 hours, or about 10 minutes to about 5 hours, or preferably about 10 minutes to about 3 hours, or about 10 minutes to about 2 hours, and thereafter allowed to cool. The cooling temperature and cooling rate is non-limiting and, therefore, does not constrain the process to any particular cooling equipment or staffing requirement, thereby limiting costs. The cooling may be performed to reach ambient temperature by any suitable means, including air cooling or other passive cooling techniques, in some aspects. In various aspects, cooled high temperature TMCP steel has not been formed into a pipe (e.g., welded) prior to performing the final heat treatment process, after allowing further cooling from the final heat treatment, it may thereafter be formed into a tubular linepipe (e.g., welded).

The TMCP process described herein may be applied to steel compositions manufactured into linepipe by tubular welding techniques, alone or in combination with the final heat treatment process of the present disclosure. In various other aspects, the present disclosure is equally applicable to steel compositions manufactured into linepipe by tubular seamless techniques (“seamless formed steel”), requiring the combination of both the TMCP process (e.g., hot deformation process) and the final heat treatment processes described herein. As used herein, the term “seamless formed carbon steel” or “seamless formed steel,” and grammatical variants thereof, refers to a formed steel, as described hereinabove. Seamless formed steel is shaped during the TMCP processes into a tubular pipe having a hollow section and no seams (as opposed to welded pipe comprising seam welds) and quenched. When the present disclosure provides for a seamless formed steel, the final heat treatment process is mandatory and not optional.

Seamless formed steel is manufactured using TMCP by reheating a steel billet at a RHT. While hot, the steel billet is pierced through the center (e.g., with a rotary piercer and a set of roller arrangements to maintains the piercer at the center of the billet). The billet is rolled and stretched until it meets a desired length, diameter, and wall thickness. For example, the inner diameter of the seamless formed steel may be approximately equivalent to the outer diameter of the rotary piercer, and the outer diameter of the seamless formed steel may be controlled using an external roller arrangement. By controlling the inner and outer diameter of the seamless formed steel, the wall thickness is also controlled.

As with welded formed steel, traditional seamless formed steel is manufactured using traditional TMCP utilizing relatively low RHT (e.g., equal to or less than or equal to 1150° C.) to reheat the billet and austenitize and relatively low FRT (e.g., less than or equal to 900° C.) to roll and stretch the billet, including one or more hot deformation steps at these relatively low temperatures, along with relatively short interpass times (i.e., time between two consecutive deformation steps) to minimize austenite grain growth, and provide fine ferrite/bainite microstructure, as described hereinabove.

Differently, the present disclosure provides for manufacturing seamless formed steel utilizing the high temperature TMCP process to provide coarser (or relatively coarser compared to traditional TMCP processes for forming seamless formed steel) austenite grain sizes that promote the formation of lath bainite, as described hereinabove.

The present disclosure provides for seamless steel formed using the high temperature TMCP process and the additional (mandatory) heat treatment described hereinabove, with an interim austenitization treatment (mandatory) step therebetween.

As used herein, the term “interim austenitization treatment process,” and grammatical variants thereof, refers to a non-deforming steel heat treatment held at a temperature above Ac3 (i.e., the critical temperature at which free ferrite is completely transformed into austenite) for a period of time, followed by quenching. It is to be understood that while the present disclosure discusses the interim austenitization treatment process with reference to the formation of seamless steel, the interim austenitization treatment may equally be applied to form any type of linepipe without limitation.

Unlike traditional austenitization treatments, which are typically performed only a several degrees above Ac3 (e.g., Ac3 to less than Ac3+50° C.) to minimize austenite growth in favor of finer microstructure after quenching, the interim austenitization treatments of the present disclosure employ higher temperatures to coarsen austenite grain size to promote lath bainite microstructure, which may be particularly beneficial for the alternative low-manganese and/or low-carbon content carbon steel compositions of the present disclosure. In one or more aspects, steel formed after completion of a high temperature TMCP process (or other formed steel process) may be subsequently interim austenitization treatment processed (i.e., prior to the final heat treatment described herein) at a temperature in the range of about (Ac3+50° C.) (i.e., the Ac3 temperature plus 50° C.) to about (Ac3+200° C.), such as in the range of about (Ac3+50° C.) to about (Ac3+150° C.), or in the range of about (Ac3+75° C.) to about (Ac3+100° C.), encompassing any value and subset therebetween.

In one or more aspects, the heating rate for the interim austenitization treatment may be in the range of about 0.1° C. per second (° C./sec) to about 100° C./sec, encompassing any value and subset therebetween; such rates may depend on a number of factors including, but not limited to, the type of heating used (e.g., induction heating v. non-induction heating).

Upon reaching the desired temperature during the interim austenitization treatment, it is maintained stable for a predetermined length of time, such as in the range of greater than about one (1) minute to about 10 hours, encompassing any value and subset therebetween, such as about minutes to about 8 hours, or about 10 minutes to about 6 hours, or about 10 minutes to about 5 hours, or preferably about 10 minutes to about 3 hours, or about 10 minutes to about 2 hours, and thereafter followed by quenching. Quenching includes accelerated cooling, wherein a fluid is used to increase cooling rate (as opposed to air cooling); other quenching methods may also be employed, without departing from the scope of the present disclosure.

After the interim austenitization treatment, the carbon steel may be thereafter processed according to the final heat treatment process described hereinabove. When the formed carbon steel is a seamless pipe, the interim austenitization treatment and the final heat treatment process are both mandatory. When the formed carbon steel is a non-seamless pipe (e.g., a plate formed into a tubular linepipe, such as by welding), the interim austenitization treatment and the final heat treatment are both optional after the high temperature TMCP of the present disclosure, alone or in combination.

The present disclosure provides, among others, the following aspects, each of which may be considered as optionally including any alternate aspects.

Clause 1. A carbon steel composition comprising: manganese in an amount of equal to or less than about 1.6% by weight or less than about 1.3% by weight of the carbon steel composition.

Clause 2. The carbon steel composition of Clause 1, wherein the manganese is in a range of about 0.6 wt. % to about 1.6 wt. % or about 0.6% to about 1.3% by weight of the carbon steel composition.

Clause 3. The carbon steel composition of Clause 1, further comprising one or more elements selected from the group consisting of carbon, phosphorous, sulfur, silicon, aluminum, chromium, molybdenum, niobium, titanium, nitrogen, calcium, nickel, vanadium, boron, and any combination thereof.

Clause 4. The carbon steel composition of Clause 3, wherein the carbon steel comprises at least nickel in an amount in the range of about 0.15% to about 1.0% by weight of the carbon steel composition.

Clause 5. The carbon steel composition of Clause 3, wherein the carbon steel comprises at least carbon in an amount in the range of about 0.01% to about 0.15% by weight of the carbon steel composition.

Clause 7. The carbon steel composition of Clause 5, wherein the carbon is in a range of about 0.03% to about 0.12% by weight of the carbon steel composition.

Clause 8. The carbon steel composition of Clause 3 to Clause 7, wherein the carbon steel composition comprises at least niobium in an amount of about 0.02% to about 0.10% by weight of the carbon steel composition.

Clause 9. The carbon steel composition of Clause 3 to Clause 8, wherein the carbon steel composition comprises at least vanadium in an amount of about 0.02% to about 0.10% by weight of the carbon steel composition.

Clause 10. The carbon steel composition of Clause 3 to Clause 9, wherein the carbon steel composition comprises at least niobium and vanadium, and the combined amount of niobium and vanadium is in an amount of about 0.02% to about 0.15% by weight of the carbon steel composition.

Clause 11. The carbon steel composition of Clause 3 to Clause 10, wherein the carbon steel comprises at least boron in an amount in the range of about 0.0005% to about 0.003% by weight of the carbon steel composition.

Clause 12. The carbon steel composition of Clause 3 to Clause 11, wherein the carbon steel comprises at least chromium in an amount in the range of about 0.1% to about 0.75% or about to about 0.5% by weight of the carbon steel composition.

Clause 13. The carbon steel composition of Clause 3 to Clause 12, wherein the carbon steel comprises at least molybdenum in an amount in the range of about 0.1% to about 0.5% by weight of the carbon steel composition.

Clause 14. The carbon steel composition of Clause 3 to Clause 13, wherein the carbon steel comprises at least titanium in an amount in the range of about 0.005% to about 0.1% by weight of the carbon steel composition.

Clause 15. A carbon steel composition comprising: carbon in an amount of equal to or less than about 0.025% by weight of the carbon steel composition.

Clause 16. The carbon steel composition of Clause 15, wherein the carbon is in a range of about 0.01% to about 0.025% by weight of the carbon steel composition.

Clause 17. The carbon steel composition of Clause 15, further comprising one or more elements selected from the group consisting of manganese, phosphorous, sulfur, silicon, aluminum, chromium, molybdenum, niobium, titanium, nitrogen, calcium, nickel, vanadium, boron, and any combination thereof.

Clause 18. The carbon steel composition of Clause 17, wherein the carbon steel comprises at least manganese in an amount of equal to or less than about 1.6% by weight or less than about 1.3% by weight of the carbon steel composition.

Clause 19. The carbon steel composition of Clause 17 or Clause 18, wherein the manganese is in a range of about 0.6% to about 1.6% or about 0.6% to about 1.3% by weight of the carbon steel composition.

Clause 20. The carbon steel composition of Clause 17 to Clause 19, wherein the carbon steel composition comprises at least niobium in an amount of about 0.02% to about 0.10% by weight of the carbon steel composition.

Clause 21. The carbon steel composition of Clause 17 to Clause 20, wherein the carbon steel composition comprises at least vanadium in an amount of about 0.02% to about 0.10% by weight of the carbon steel composition.

Clause 22. The carbon steel composition of Clause 17 to Clause 21, wherein the carbon steel composition comprises at least niobium and vanadium, and the combined amount of niobium and vanadium is in an amount of about 0.02% to about 0.15% by weight of the carbon steel composition.

Clause 23. The carbon steel composition of Clause 17 to Clause 22, wherein the carbon steel comprises at least nickel in an amount in the range of about 0.15% to about 1.0% by weight of the carbon steel composition.

Clause 24. The carbon steel composition of Clause 17 to Clause 23, wherein the carbon steel comprises at least boron in an amount in the range of about 0.0005% to about 0.003% by weight of the carbon steel composition.

Clause 25. The carbon steel composition of Clause 17 to Clause 24, wherein the carbon steel comprises at least chromium in an amount in the range of about 0.1% to about 0.75% or about to about 0.5% by weight of the carbon steel composition.

Clause 26. The carbon steel composition of Clause 17 to Clause 25, wherein the carbon steel comprises at least molybdenum in an amount in the range of about 0.1% to about 0.5% by weight of the carbon steel composition.

Clause 27. The carbon steel composition of Clause 17 to Clause 26, wherein the carbon steel comprises at least titanium in an amount in the range of about 0.005% to about 0.1% by weight of the carbon steel composition.

To facilitate a better understanding of the aspects of the present disclosure, the following examples of preferred or representative are given. In no way should the following examples be read to limit, or to define, the scope of the present disclosure.

EXAMPLES Example 1—Low Magnesium Carbon Steel Comprising Niobium

In this example, carbon steel composition comprising low-manganese content, as defined herein, were prepared and tested for mechanical properties and SSC resistance, including the effect of the high RHT/FRT TMCP and final heat treatment methods of the present disclosure.

Four (4) carbon steel ingots (ID1-ID4) were prepared, each having the compositions listed in Table 1, based on weight percent (wt. %). ID1 represents a standard, commercially available API-5L sour grade carbon steel composition used for linepipe; ID2-ID4 represent low-manganese carbon steel compositions.

TABLE 1 ID1 ID2 ID3 ID4 C 0.064 0.047 0.043 0.042 Mn 1.41 0.98 1.03 0.98 P 0.010 0.009 0.008 0.008 S 0.001 0.001 0.005 0.005 Si 0.261 0.378 0.372 0.394 Al 0.008 0.032 0.039 0.031 Cr 0.21 0.19 0.20 0.20 Mo 0.10 0.10 0.10 0.10 Nb 0.030 0.029 0.029 0.030 Ti 0.014 0.014 0.009 0.009 N 0.0049 0.0030 0.0025 0.0024 Ca 0.0008 0.0020 0.0010 0.0009

Carbon steels were prepared using the compositions of ID1-ID4 using vacuum induction melting (VIM), producing ingots of approximately 50 kg having a width of about 125 millimeters (mm) and a thickness of about 125 mm. The ingots were each TMCP treated according to either a traditional TMCP method (represented by ID1 and ID2) or a TMCP method according to various aspects of the present disclosure (represented by ID3 and ID4). Generally, the ingots were reheated at a reheating temperature (RHT), and thereafter control hot rolled to a final thickness of about 20-25 mm at a particular finishing rolling temperature (FRT). Each of ID1-ID4 underwent between eleven (11) and thirteen (13) hot rolling passes to achieve the final thickness. The FRT was selected at a temperature greater than Ara, the temperature at which austenite begins phase transformation onset during cooling (transformation to ferrite and/or bainite) and with various interpass durations, including at least one extended interpass duration, as defined hereinabove (temperature and time), followed by accelerated cooling control (ACC) at a cooling rate of greater than about 5° C./sec (e.g., in the range of about 5° C./sec to about 50° C./sec, or about 20° C./sec to about 50° C./sec) to ambient temperature. The specific TMCP parameters for this Example are provided in Table 2; actual extended interpass temperatures ranged between 950° C. to 1150° C. due to production conditions and equipment requirements.

TABLE 2 RHT (° C.) FRT (° C.) Extend Interpass ID1 1150 900 No ID2 1150 925 No ID3 1230 900 Yes ID4 1230 860 Yes

The TMCP treated ID1-ID4 carbon steels were evaluated for mechanical properties and SSC resistance. Mechanical properties, including yield strength (YS) measured in kip per square inch (ksi), tensile strength (TS) measured in ksi, and percent elongation (EL), were tested according to sub-size round bar with 1″ gauge length of ASTM-E8.

SSC resistance was evaluated using double cantilever beam (DCB) testing, using standard NACE TM0177 Method D (2016) test method, which is based on crack-arrest fracture mechanics (or decaying K loading) for high strength carbon steels. DCB testing measures the resistance of environmental cracking (EC) propagation (typically aligned with the rolling direction of steel plates or the longitudinal direction of tubular products), expressed in terms of a critical stress intensity factor (K_(ISSC)) measured in ksi-in^(0.5). DCB testing permits quantitative measurement of the SSC arrest toughness (K_(ISSC)); each K_(ISSC) reported herein is an average K_(ISSC) (of all observed cracks). DCB testing was performed by exposing samples of ID1-ID4 in the standard NACE A sour solution, representing a severe sour condition, for 14 days (336 hours). The standard NACE A sour solution comprises 5 wt. % sodium chloride and 0.5 wt. % acetic acid saturated with 100% hydrogen sulfide gas at 1 bar and an initial pH of 2.7. The standard NACE A sour solution belongs to the severe sour region (Region 3) in the NACE diagram of pH2S vs. pH per NACE MR-0175. The range of average K_(ISSC) with reference to the alternative steel compositions of the present disclosure is accordingly with reference to NACE MR-0175 with NACE A conditions at 1 bar, unless otherwise specified.

The mechanical and SSC resistance results of hot rolled materials are provided in Table 3.

TABLE 3 YS (ksi) TS (ksi) EL (%) K_(ISSC) (ksi-in^(0.5)) ID1 67.0 90.1 27.5 35.0 ID2 57.4 88.1 32.5 37.7 ID3 60.3 85.5 36.5 41.4 ID4 61.0 87.0 32.5 38.0

The results indicate that each of the low-manganese concentration carbon steels of ID2-ID4 exhibit greater SSC resistances compared to the commercially available, higher manganese concentration carbon steel of ID1, regardless of whether the formed carbon steel was prepared according to traditional TMCP (ID2) or the high RHT/FRT TMCP of the present disclosure (ID3 and ID4). It is noted that the low-manganese concentration carbon steels prepared according to the TMCP methods of the present disclosure (including extended interpass time) demonstrated even greater improved SSC resistance compared to traditional TMCP methods. A comparison of ID3 and ID4, further indicates that a higher FRT may enhance the SSC resistance, as ID3 was processed with a higher FRT compared to ID4. Moreover, the mechanical properties of the low-manganese produced steels are suitable for use as linepipe. Accordingly, in some aspects, the steel plates formed in accordance with the methods of the present disclosure may have a K_(ISSC) of greater than about 35 ksi-in^(0.5), such as in the range of greater than about 35 ksi-in 0.5 to about 60 ksi-in^(0.5), or about 36 ksi-in^(0.5) to about 60 ksi-in^(0.5), encompassing any value and subset therebetween.

To evaluate the effect of the final heat treatment step described in the present disclosure, heat (HT) was applied to each of ID1-ID4 and the metals were thereafter tested for mechanical properties and SSC resistance, as described above. ID1 was exposed to a final heat treatment at 625° C. and ID2-ID4 were exposed to a final heat treatment at 575° C., each for two (2) hours and allowed to cool to room temperature. The results are shown in Table 4.

TABLE 4 YS (ksi) TS (ksi) EL (%) K_(ISSC) (ksi-in^(0.5)) ID1 78.0 89.8 32.0 42.6 ID2 71.5 87.0 36.0 45.5 ID3 72.5 85.3 33.5 51.6 ID4 65.3 77.0 35.5 41.1

Compared to the results in Table 3 (without final heat treatment), each of ID1-ID4 exhibited enhanced SSC resistance after final heat treatment, as shown in Table 4. ID1 improved by about 22%; ID2 improved by about 21%; ID3 improved by about 25%; and ID4 improved by about 8%. Moreover, the final heat treatment step generally resulted in increased yield strength and elongation and, if at all, comparable or marginally decreased tensile strength while improving K_(ISSC). Further, the final heat treatment reveals a synergistic effect when combined with the high temperature TMCP process of the present disclosure, including, if not an even more pronounced synergistic effect at higher temperatures within the ranges of the TMCP process of the present disclosure (e.g., higher FRT).

Accordingly, in some aspects, the final heat treatment methods of the present disclosure may provide comparatively (e.g., to identical compositions without the final heat treatment) enhanced SSC resistance by greater than about 4%, including greater than about 5%, such as in the range of greater than about 5% to about 45%, or greater than about 5% to about 40%, encompassing any value and subset therebetween, including the alternative carbon steel compositions of the present disclosure as well as traditional carbon steels.

Without being bound by theory, it is further believed, as described above, that the amount of Nb present in the low-manganese compositions ID2-ID4 (0.029 wt. %, 0.029 wt. %, and 0.030 wt. %, respectively) in combination with the final heat treatment may at least partially contribute to the SSC resistance by forming fine precipitates that trap hydrogen. Indeed, the increase in SSC resistance upon final heat treatment may further be attributable to other metallurgical changes, such as tempering on M/A constituents (e.g., disassociation of M/A constituents), reduction of dislocation density, and the like, and any combination thereof.

Therefore, the low-manganese carbon steel compositions of the present disclosure are suitable for use as linepipe and meet required industry standards.

Example 2—Low Manganese Carbon Steels Comprising Niobium and Vanadium

In this example, carbon steel composition comprising low-manganese content, as defined herein, were prepared and tested for mechanical properties and SSC resistance.

Five (5) carbon ingots were prepared according to the methods described in Example 1, having the compositions listed in Table 5 and the processing parameters listed in Table 6, using either a traditional TMCP method (represented by ID5 and ID7) or a TMCP method according to various aspects of the present disclosure (represented by ID6, ID9, and ID10). ID5-ID7 represent standard, commercially available API-5L sour grade carbon steel compositions used for linepipe; ID9 and ID10 represent low-manganese carbon steel compositions. Each of ID5-ID10 include niobium and vanadium.

TABLE 5 ID5 ID6 ID7 ID9 ID10 C 0.044 0.043 0.039 0.041 0.043 Mn 1.37 1.36 1.39 1.00 1.01 P 0.007 0.008 0.008 0.008 0.008 S 0.005 0.005 0.005 0.004 0.004 Si 0.257 0.243 0.253 0.403 0.418 Al 0.031 0.024 0.037 0.032 0.032 Cr 0.20 0.20 0.21 0.20 0.20 Mo 0.10 0.10 0.11 0.11 0.11 Nb 0.029 0.029 0.031 0.031 0.032 Ti 0.008 0.009 0.009 0.009 0.010 V 0.029 0.031 0.059 0.030 0.063 N 0.0028 0.0028 0.0035 0.0029 0.0028 Ca 0.0007 0.0006 0.0006 0.0009 0.0011

TABLE 6 RHT (° C.) FRT (° C.) Extend Interpass ID5 1150 885 No ID6 1230 905 Yes ID7 1150 885 No ID9 1230 900 Yes ID10 1230 900 Yes

FIGS. 3A-3C show representative micrographs of the conventional carbon steels ID5-ID7, respectively, formed according to either ltTMCP or htTMCP of this example; FIGS. 3E and 3F show representative micrographs of low-manganese carbon steels comprising niobium and vanadium ID9 and ID10, respectively, formed according to htTMCP of this example. As shown, each of the carbon steels have bainite matrices (granular bainite) with M/A constituents and ferrite. As shown in FIGS. 3D and 3E, the low-manganese carbon steels comprising niobium and vanadium of ID9 and ID10 promote the formation of ferrite with reduced M/A constituents.

The ID5-ID10 carbon steels were evaluated for mechanical properties and SSC resistance (using NACE A) as described in Example 1. The results are reported in Table 7.

TABLE 7 YS (ksi) TS (ksi) EL (%) K_(ISSC) (ksi-in^(0.5)) ID5 57.5 90.5 36.0 40.4 ID6 65.0 88.0 32.5 38.6 ID7 61.5 89.8 33.0 38.7 ID9 59.0 87.3 36.5 41.6 ID10 59.5 89.5 35.0 41.1

The results indicate that each of the low-manganese concentration carbon steels of ID9 and ID10 exhibit greater SSC resistances compared to the commercially available, higher manganese concentration carbon steels of ID5-ID7, regardless of whether the formed carbon steel was prepared according to traditional TMCP (ID5 and ID7) or the high RHT/FRT TMCP of the present disclosure (ID6). Moreover, the mechanical properties of the low-manganese produced steels are suitable for use as linepipe. Accordingly, in some aspects, the steel plates formed in accordance with the methods of the present disclosure may have a K_(ISSC) of greater than about 35 ksi-in^(0.5), such as in the range of greater than about 35 ksi-in^(0.5) to about 60 ksi-in^(0.5), or about 36 ksi-in^(0.5) to about 60 ksi-in^(0.5), encompassing any value and subset therebetween.

To evaluate the effect of the final heat treatment step described in the present disclosure, heat (HT) was applied to each of ID5-ID10 and the metals were thereafter tested for mechanical properties and SSC resistance, as described above. ID5-ID10 were exposed to a final heat treatment at 575° C. and the results are shown in Table 8; ID5-ID10 were exposed to a final heat treatment at 675° C. and the results are shown in Table 9. Where a “-” is shown, the particular test was not performed.

TABLE 8 YS (ksi) TS (ksi) EL (%) K_(ISSC) (ksi-in^(0.5)) ID5 73.5 86.3 30.0 — ID6 78.5 90.8 29.0 47.4 ID7 75.0 86.8 33.0 49.0 ID9 72.3 84.3 30.0 50.9 ID10 75.3 88.5 34.5 48.8

TABLE 9 YS (ksi) TS (ksi) EL (%) K_(ISSC) (ksi-in^(0.5)) ID5 69.5 80.0 31.5 — ID6 69.5 81.3 30.0 48.5 ID7 70.5 81.1 30.5 — ID9 68.5 79.8 33.5 44.9 ID10 71.8 81.5 33.5 43.2

Compared to the results in Table 7 (without final heat treatment), each of ID6-ID10 exhibited enhanced SSC resistance after final heat treatment of 575° C., as shown in Table 8. Moreover, the final heat treatment step generally resulted in increased yield strength and elongation and, if at all, comparable or marginally decreased tensile strength while improving K_(ISSC). ID6 improved by about 23%; ID7 improved by about 27%; ID9 improved by about 22%; and ID10 improved by 19%. Accordingly, as consistent with Example 1, the final heat treatment methods of the present disclosure may provide comparatively (e.g., to identical compositions without the final heat treatment) enhanced SSC resistance by greater than about 4%, including the alternative carbon steel compositions of the present disclosure as well as traditional carbon steels.

Similarly, compared to the results in Table 7 (without final heat treatment), each of ID6, ID9, and ID10 exhibited enhanced SSC resistance after final heat treatment of 675° C., as shown in Table 9.

Similar to Example 1, and without being bound by theory, it is further believed, as described above, that the amount of V and Nb present in the low-manganese compositions ID9 and ID10 in combination with the final heat treatment may at least partially contribute to the SSC resistance by forming fine precipitates that trap hydrogen. Indeed, the increase in SSC resistance upon final heat treatment may further be attributable to other metallurgical changes, such as tempering on M/A constituents (e.g., disassociation of M/A constituents), reduction of dislocation density, and the like, and any combination thereof.

Therefore, the low-manganese carbon steel compositions of the present disclosure are suitable for use as linepipe and meet required industry standards.

Example 3—Low-Carbon Carbon Steels Comprising Niobium and Vanadium

In this example, carbon steel composition comprising low-carbon content, as defined herein, were prepared and tested for mechanical properties and SSC resistance. The low-carbon carbon steels of this Example are compared to the conventional carbon steels ID5-ID7 of Example 2.

Two (2) carbon ingots were prepared according to the methods described in Example 1, having the compositions listed in Table 10 and the processing parameters listed in Table 11, using a TMCP method according to various aspects of the present disclosure. Each of low-carbon carbon steel compositions ID11-ID12 include niobium and vanadium.

TABLE 10 ID11 ID12 C 0.013 0.015 Mn 1.39 1.37 P 0.001 0.008 S 0.001 0.002 Si 0.378 0.240 Al 0.036 0.027 Cr 0.21 0.21 Mo 0.11 0.11 Nb 0.029 0.029 Ti 0.009 0.008 V 0.029 0.057 N 0.0033 0.0030 Ca 0.0012 0.0007

TABLE 11 RHT (° C.) FRT (° C.) Extend Interpass ID11 1230 960 Yes ID12 1230 950 Yes

FIGS. 4A and 4B show representative micrographs of low-carbon carbon steels comprising niobium and vanadium ID11 and ID12, respectively, formed according to htTMCP of this example. As compared to FIGS. 3A-3D, the low-carbon carbon steels comprising niobium and vanadium of ID11 and ID12 promote the formation of ferrite with reduced M/A constituents.

The ID11 and ID12 carbon steels were evaluated for mechanical properties and SSC resistance (using NACE A) as described in Example 1. The results are reported in Table 12.

TABLE 12 YS (ksi) TS (ksi) EL (%) K_(ISSC) (ksi-in^(0.5)) ID11 59.0 78.8 32.5 36.4 ID12 60.0 79.5 34.0 39.0

The results indicate that each of the low-carbon concentration carbon steels of ID11 and ID12 exhibit comparable SSC resistances compared to the commercially available, higher carbon concentration carbon steels of ID5-ID7 (Example 2), including those prepared using the high RHT/FRT TMCP of the present disclosure (ID6 of Example 2). Moreover, the mechanical properties of the low-carbon produced steels are suitable for use as linepipe, exhibiting comparable mechanical properties to conventional carbon steels. Accordingly, in some aspects, the steel plates formed in accordance with the methods of the present disclosure may have a K_(ISSC) of greater than about 35 ksi-in^(0.5), such as in the range of greater than about 35 ksi-in^(0.5) to about 60 ksi-in^(0.5), or about 36 ksi-in^(0.5) to about 60 ksi-in^(0.5), encompassing any value and subset therebetween.

To evaluate the effect of the final heat treatment step described in the present disclosure, heat (HT) was applied to each of ID1-ID12 and the metals were thereafter tested for mechanical properties and SSC resistance, as described above. ID11-ID12 were exposed to a final heat treatment at 575° C. and the results are shown in Table 13; ID11-ID12 were exposed to a final heat treatment at 675° C. and the results are shown in Table 14. Where a “-” is shown, the particular test was not performed.

TABLE 13 YS (ksi) TS (ksi) EL (%) K_(ISSC) (ksi-in^(0.5)) ID11 67.0 80.0 33.5 41.4 ID12 71.3 82.5 32.0 —

TABLE 14 YS (ksi) TS (ksi) EL (%) K_(ISSC) (ksi-in^(0.5)) ID11 67.0 80.0 33.5 48.6 ID12 69.3 80.0 32.0 42.1

Compared to the results in Table 12 (without final heat treatment), ID11 exhibited enhanced SSC resistance after final heat treatment of 575° C., as shown in Table 8. Moreover, the final heat treatment step generally resulted in increased yield strength and tensile strength and, if at all, comparable or marginally decreased elongation while improving K_(ISSC). ID11 improved by about 14%. Accordingly, as consistent with Example 1 and Example 2.

Similarly, compared to the results in Table 12 (without final heat treatment), ID12 exhibited enhanced SSC resistance after final heat treatment of 675° C., as shown in Table 14.

Similar to Example 2, having low-manganese carbon steel compositions with Nb and V, and without being bound by theory, it is further believed, as described above, that the amount of Nb and V present in the low-carbon compositions ID11 and ID12 in combination with the final heat treatment may at least partially contribute to the SSC resistance by forming fine precipitates that trap hydrogen by promoting the precipitation of NbC and/or VC. Indeed, the increase in SSC resistance upon final heat treatment may further be attributable to other metallurgical changes, such as tempering on M/A constituents (e.g., disassociation of M/A constituents), reduction of dislocation density, and the like, and any combination thereof.

Therefore, the low-carbon carbon steel compositions of the present disclosure are suitable for use as linepipe and meet required industry standards.

Example 4—Low-Manganese and Low-Carbon Carbon Steels Comprising Niobium and Vanadium

In this example, carbon steel composition comprising both low-manganese and low-carbon content, as defined herein, were prepared and tested for mechanical properties and SSC resistance.

Four (4) carbon ingots were prepared according to the methods described in Example 1, having the compositions listed in Table 15 and the processing parameters listed in Table 16, using a traditional TMCP method. ID 13 represents a low-manganese carbon steel composition prepared according to aspects of the present disclosure; ID13-ID16 represent combination low-manganese and low-carbon carbon steel compositions prepared according to aspects of the present disclosure. Each of ID13-ID16 include niobium and vanadium.

TABLE 15 ID13 ID14 ID15 ID16 C 0.048 0.015 0.013 0.012 Mn 0.59 0.60 0.99 1.00 P 0.008 0.008 0.008 0.008 S 0.0010 0.0010 0.0009 0.0010 Si 0.400 0.405 0.402 0.403 Al 0.038 0.037 0.030 0.027 Cr 0.202 0.204 0.208 0.211 Mo 0.105 0.106 0.108 0.108 Nb 0.026 0.029 0.029 0.029 Ti 0.008 0.009 0.009 0.010 V 0.052 0.054 0.029 0.057 N 0.0028 0.0032 0.0027 0.0030 Ca 0.0008 0.0013 0.0011 0.0011

TABLE 16 RHT (° C.) FRT (° C.) Extend Interpass ID13 1230 950 Yes ID14 1230 960 Yes ID15 1230 960 Yes ID16 1230 960 Yes

FIG. 5A shows a representative micrograph of a low-manganese carbon steel comprising niobium and vanadium of the present disclosure; FIGS. 5B-5D show representative micrographs of combination low-manganese and low-carbon carbon steels comprising niobium and vanadium of the present disclosure. As shown, each of the carbon steels have ferrite matrices with M/A constituents. The additional lower carbon contents of ID14-ID16 even further reduces the M/A constituents.

The ID13-ID16 carbon steels were evaluated for mechanical properties and SSC resistance (using NACE A) as described in Example 1. The results are reported in Table 17.

TABLE 17 YS (ksi) TS (ksi) EL (%) K_(ISSC) (ksi-in^(0.5)) ID13 54.5 82.3 33.0 35.6 ID14 49.3 68.8 42.0 36.2 ID15 51.3 73.0 37.0 35.6 ID16 55.5 76.0 35.5 35.9

The results indicate that each of the low-manganese/low-carbon concentration carbon steels of ID14 and ID16 exhibit comparable SSC resistances compared to the low-manganese only (higher carbon concentration) carbon steel of ID 13. Moreover, the mechanical properties of the low-manganese/low-carbon produced steels are suitable for use as linepipe, exhibiting comparable mechanical properties to conventional carbon steels as provided herein, as well as the low-manganese only carbon steel of ID13. Accordingly, in some aspects, the steel plates formed in accordance with the methods of the present disclosure may have a K_(ISSC) of greater than about ksi-in^(0.5), such as in the range of greater than about 35 ksi-in^(0.5) to about 60 ksi-in^(0.5), or about 36 ksi-in^(0.5) to about 60 ksi-in^(0.5), encompassing any value and subset therebetween.

To evaluate the effect of the final heat treatment step described in the present disclosure, heat (HT) was applied to each of ID13-ID16 and the metals were thereafter tested for mechanical properties and SSC resistance, as described above. ID13-ID16 were exposed to a final heat treatment at 575° C. and the results are shown in Table 18; ID13-ID16 were exposed to a final heat treatment at 675° C. and the results are shown in Table 19. Where a “—” is shown, the particular test was not performed.

TABLE 18 YS (ksi) TS (ksi) EL (%) K_(ISSC) (ksi-in^(0.5)) ID13 66.3 81.3 34.5 — ID14 52.3 68.0 37.0 37.5 ID15 61.8 73.8 36.5 — ID16 62.0 74.3 36.0 —

TABLE 19 YS (ksi) TS (ksi) EL (%) K_(ISSC) (ksi-in^(0.5)) ID13 65.8 79.8 32.5 45.6 ID14 58.8 69.0 38.5 42.9 ID15 67.3 78.3 33.0 47.4 ID16 68.5 80.0 32.0 48.4

Notably, ID14 was also exposed to a final heat treatment at 500° C., resulting in a YS of 53.8 ksi, TS of 68.8 ksi, EL of 36.0%, and K_(ISSC) of 35.5 ksi-in^(0.5).

Compared to the results in Table 17 (without final heat treatment), ID14 exhibited enhanced SSC resistance after final heat treatment of 575° C., as shown in Table 18. Moreover, the final heat treatment step generally resulted in increased yield strength and, if at all, comparable or marginally decreased tensile strength and elongation while improving K_(ISSC). ID14 improved by about 4%. Accordingly, as consistent with Examples 1, 2, and 3, the final heat treatment methods of the present disclosure may provide comparatively (e.g., to identical compositions without the final heat treatment) enhanced SSC resistance by greater than about 4%, including the alternative carbon steel compositions of the present disclosure as well as traditional carbon steels.

Similarly, compared to the results in Table 17 (without final heat treatment), ID13-ID16 exhibited enhanced SSC resistance after final heat treatment of 675° C., as shown in Table 19. Notably, at least with reference to ID14, the increase in SSC resistance was greater at the higher temperature of 675° C. compared to 525° C., which may be attributable the combined low-carbon and low-manganese content.

Similar to the previous, and without being bound by theory, it is further believed, as described above, that the amount of Nb and V present in the low-carbon compositions ID13 and ID16 in combination with the final heat treatment may at least partially contribute to the SSC resistance by forming fine precipitates that trap hydrogen, promoting the precipitation of NbC and/or VC. Indeed, the increase in SSC resistance upon final heat treatment may further be attributable to other metallurgical changes, such as tempering on M/A constituents (e.g., disassociation of M/A constituents), reduction of dislocation density, and the like, and any combination thereof.

Therefore, the low-carbon carbon steel compositions of the present disclosure are suitable for use as linepipe and meet required industry standards.

Example 5—Low-Manganese Carbon Steels Comprising Nickel

In this example, carbon steel compositions comprising both low-manganese with Ni content, as defined herein, were prepared and tested for mechanical properties and SSC resistance.

Two carbon ingots were prepared according to the methods described in Example 1, having the compositions listed in Table 19 and the processing parameters listed in Table 20. ID 17-18 represents low-manganese carbon steel composition with Ni content prepared according to aspects of the present disclosure.

TABLE 19 ID17 ID18 C 0.038 0.036 Mn 0.98 0.98 P 0.008 0.007 S 0.005 0.005 Si 0.37 0.39 Al 0.03 0.02 Cr 0.21 0.20 Mo 0.10 0.10 Ni 0.26 0.78 Nb 0.029 0.031 Ti 0.008 0.009 N 0.0027 0.0030 Ca 0.0010 0.0010

TABLE 20 RHT (° C.) FRT (° C.) Extend Interpass ID17 1230 900 Yes ID18 1230 900 Yes

The ID17 and ID18 carbon steels were evaluated for mechanical properties and SSC resistance (using NACE A) as described in Example 1. The results are reported in Table 21. The results indicate that the carbon steels comprising alternative low-manganese content and nickel exhibit greater SSC resistance compared to commercially available carbon steels, such as ID1.

TABLE 21 YS (ksi) TS (ksi) EL (%) K_(ISSC) (ksi-in^(0.5)) ID17 59.8 88.3 27.5 41.9 ID18 64.8 91.0 31.5 38.9

To evaluate the effect of Ni after final heat treatment, final heat treatment was applied at different temperatures to three (3) separate samples of ID17 and a sample of ID18 and thereafter tested for mechanical properties and SSC resistance, as described above. Each sample was exposed to one or more temperatures (as indicated in Table 4), and cooled to room temperature prior to testing. Testing results are shown in Table 22. Where a “-” is shown, the particular test was not performed.

TABLE 22 YS (ksi) TS (ksi) EL (%) K_(ISSC) (ksi-in^(0.5)) ID17 550 — — — 52.3 ID17 575 — — — 54.5 ID17 625 71.5 83 34 51.7 ID18 625 75.75 87.5 32 50.4

The carbon steel of ID17 was heat treated at 550, 575 and 625° C., and ID18 heat treated at 625° C. The heat treatment of both carbon steels (ID17 and ID18) enhanced SSC resistance compared to the results in Table 21, regardless of the particular temperature. ID17 showed the best result at temperature of 575° C. In addition, the final heat treatment generally resulted in the increase in yield strength and elongation, and comparable or marginally decreased tensile strength. As with the prior carbon steels of the present disclosure, improved or additional SSC resistance may be optimized based on selected temperature for the final temperature treatment.

Example 6—Low-Manganese Carbon Steels Comprising Carbon, Boron, Niobium, Chromium, Molybdenum and Nickel Produced by Tubular Seamless Technique

In this example, carbon steel compositions were assessed in order to evaluate tensile properties and SSC resistance of materials processed by tubular seamless technique with various alloying elements. Nine (9) ingots were prepared according to the methods described in Example 1, having the compositions listed in Table 23. ID19-ID22 represent lower carbon/manganese carbon steel composition comprising Nb and B content prepared according to aspects of the present disclosure. ID23-ID27 represent the low-manganese carbon steel compositions with the combination of higher C/Cr/Mo/Nb/Ni, prepared according to aspects of the present disclosure.

TABLE 23 ID19 ID20 ID21 ID22 ID23 ID24 ID25 ID26 ID27 C 0.06 0.05 0.05 0.04 0.10 0.10 0.10 0.06 0.08 Mn 0.80 0.97 0.99 1.27 1.24 1.20 1.26 1.22 1.22 P 0.009 0.007 0.008 0.007 0.007 0.007 0.007 0.01 0.01 S 0.004 0.005 0.005 <0.005 <0.005 <0.005 <0.005 <0.005 <0.005 Si 0.39 0.39 0.39 0.38 0.38 0.40 0.41 0.41 0.41 Al 0.034 0.037 0.027 0.030 0.040 0.030 0.030 0.030 0.030 Cr 0.20 0.21 0.20 0.21 0.21 0.37 0.39 0.36 0.36 Mo 0.11 0.10 0.10 0.10 0.10 0.19 0.20 0.24 0.24 Ni — — 0.26 — — — 0.77 — — Nb 0.030 0.031 0.032 0.060 0.060 0.050 0.060 0.090 0.090 Ti 0.013 0.009 0.008 — — — — — — V — 0.052 — — — — — — — B 0.0019 0.0026 0.0028 0.0005 — — — — — N 0.0031 0.0028 0.0028 0.0030 0.0030 0.0030 0.0030 NA NA Ca 0.0017 0.0008 0.0008 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01

All ingots were reheated to high RHT of 1230° C., followed by multiple hot deformations with extended interpass time, FRT>900° C. and accelerated cooling according to aspects of the present disclosure. The formed carbon steels were subsequently interim austenitization treatment processed at a temperature in the range of >Ac3+50° C. followed by water quenching, and then final heat treatment processed according to the present disclosure. The process parameters of interim austenitization treatment and final heat treatment are listed in Table 24 with tensile properties and SSC resistance.

TABLE 24 Interim Austenitization Final Heat Treatment Treatment K_(ISSC) Temp. Time Temp. Time YS TS El (ksi- (° C.) (min) (° C.) (min) (ksi) (ksi) (%) in^(0.5)) ID19 1000 30 625 30 74.3 86.3 28.0 48.0 ID20 1000 30 575 30 76.0 89.3 25.0 39.8 625 30 81.5 94.8 27.0 40.6 675 30 83.3 92.5 25.5 41.8 ID21 1000 30 575 30 78.8 91.0 24.0 40.1 625 30 80.3 92.8 26.5 39.0 675 30 77.8 88.5 27.5 47.8 ID22 1000 30 575 30 79.0 90.0 27.5 49.3 625 30 84.3 94.0 27.5 44.3 675 30 78.8 88.3 28.5 47.4 ID23 1000 30 625 30 76.3 90.5 30.0 55.4 675 30 74.3 87.8 31.0 55.4 1100 30 625 30 93.3 106.0 22.0 31.1 625 180 85.0 97.8 27.0 38.2 675 30 88.0 100.3 25.0 47.4 675 60 77.0 91.0 27.0 43.6 ID24 1000 30 625 30 89.3 102.5 26.0 34.4 675 30 80.0 92.3 29.0 49.5 1100 30 625 30 94.5 109.5 23.5 29.3 625 180 88.3 101.5 24.0 38.2 675 30 91.3 105.0 24.5 27.8 675 180 97.0 106.0 23.5 39.5 ID25 1000 30 575 30 102.0 114.5 23.5 24.6 625 30 108.0 118.0 23.0 31.7 675 30 94.3 104.0 25.0 37.7 ID26 1100 30 625 30 96.0 109.5 25.5 27.9 625 180 93.8 106.5 25.0 29.7 ID27 1100 30 600 30 91.0 104.0 25.0 37.5 600 180 95.8 109.5 25.5 32.6

The carbon steel compositions shown in Table 23 provide higher strength (YS of about to about 110 ksi) due to the precipitation hardening and/or fraction of lath bainite by higher alloying content (higher hardenability). With the combination of one and/or more disclosed processes hereinabove, these carbon steel compositions have high fraction of tempered bainite with Nb precipitation, which provide a beneficial combination of higher strength and SSC resistance. ID19-ID23 show the combination of YS in the range of 74.3 ksi to 93.3 ksi and K_(ISSC) in the range of 31.1 ksi-in^(0.5) to 55.4 ksi-in^(0.5). Compared with the results of commercial linepipe grades (ID5-ID7) in Table 7, the results of ID19-ID23 exhibited higher strength with comparable (or better) SSC resistance. ID24-27 results in Table 24 show even higher YS in the range of 80 ksi to 108 ksi with K_(ISSC) of 24.6-49.5 ksi-in^(0.5) due to higher C/Nb/Cr/Mo/Ni content.

Example 7— High-Chromium and/or High-Manganese Carbon Steels Produced by Tubular Seamless Technique

In this example, carbon steel compositions with high Mn or Cr contents were assessed in order to evaluate SSC resistance of materials processed by tubular seamless technique. Six (6) ingots were prepared according to the methods described in Example 1, having the compositions listed in Table 25. ID28-ID32 have higher Mn content, in the range of 1.30-1.39%, with various alloying contents according to aspects of the present disclosure. ID33 has a higher Cr of 0.52%, with the combination of other alloy elements, prepared according to aspects of the present disclosure.

TABLE 25 ID28 ID29 ID30 ID31 ID32 C 0.05 0.04 0.04 0.05 0.04 Mn 1.38 1.39 1.38 1.39 1.30 P 0.008 0.008 0.010 0.008 0.007 S 0.005 0.005 0.000 0.002 <0.005 Si 0.25 0.25 0.24 0.25 0.37 Al 0.027 0.037 0.034 0.048 0.030 Cr 0.21 0.21 0.21 0.40 0.21 Mo 0.10 0.11 0.11 0.20 0.10 Nb 0.030 0.031 0.029 0.029 0.090 Ti 0.010 0.009 0.014 0.013 — V — 0.059 — — — B — — 0.0016 — 0.0010 N 0.0024 0.0035 0.0030 0.0031 0.0030 Ca 0.0007 0.0006 0.0010 0.0012 NA

All ingots were reheated to high RHT of 1230° C., followed by multiple hot deformations with extended interpass time, and FRT>900° C. with accelerated cooling according to aspects of the present disclosure. The formed carbon steels were subsequently interim austenitization treatment processed at a temperature of >Ac3+50° C. followed by water quenching, then final heat treatment processed according to the present disclosure. The process parameters of interim austenitization treatment and final heat treatment were listed in Table 26 with tensile properties and SSC resistance.

TABLE 26 Interim Austenitization Final Heat Treatment Treatment K_(ISSC) Temp. Time Temp. Time YS TS El (ksi- (° C.) (min) (° C.) (min) (ksi) (ksi) (%) in^(0.5)) ID28 950 30 625 30 72.8 85.3 29.0 45.6 1050 30 625 30 76.8 90.5 27.0 45.6 ID29 950 30 500 30 74.5 89.3 25.5 45.6 625 30 82.3 94.0 28.5 49.5 675 30 80.5 91.8 29.5 48.8 1050 30 500 30 72.0 88.5 26.0 41.3 625 30 92.5 104.5 25.5 45.4 675 30 90.3 101.0 26.5 43.6 ID30 1000 30 575 30 86.3 95.5 24.0 34.2 625 30 85.0 94.0 24.0 34.2 675 30 74.0 85.5 26.5 38.7 ID31 1000 30 625 30 75.0 86.3 29.0 46.6 675 30 74.0 83.8 28.5 48.2 ID32 1000 30 575 30 88.5 97.8 25.0 46.9 625 30 84.3 93.3 25.5 50.2 675 30 82.8 92.5 30.0 51.5 1100 30 600 30 96.8 108.0 24.5 25.0 600 60 100.5 110.5 24.0 25.5 ID33 1100 30 600 180 122.0 132.5 24.0 20.7 675 180 118.0 123.0 24.0 23.5

The carbon steel compositions shown in Table 25 provide high strength, YS in the range of 72 ksi to 122 ksi, due to the precipitation hardening and/or fraction of lath bainite by higher alloying content (higher hardenability). With the combination of one and/or more disclosed processes hereinabove, these carbon steel compositions have high fraction of tempered bainite with the precipitation of one and/or combination of Nb and Ti, which provide a beneficial combination of higher strength and SSC resistance.

Unless otherwise indicated, all numbers expressing quantities of ingredients, properties such as molecular weight, reaction conditions, and so forth used in the present specification and associated claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the incarnations of the present disclosure. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claim, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

One or more illustrative incarnations incorporating one or more aspects and elements are presented herein. Not all features of a physical implementation are described or shown in this application for the sake of clarity. It is understood that in the development of a physical embodiment incorporating one or more elements of the present disclosure, numerous implementation-specific decisions must be made to achieve the developer's goals, such as compliance with system-related, business-related, government-related and other constraints, which vary by implementation and from time to time. While a developer's efforts might be time-consuming, such efforts would be, nevertheless, a routine undertaking for those of ordinary skill in the art and having benefit of this disclosure.

While compositions and methods are described herein in terms of “comprising” various components or steps, the compositions and methods can also “consist essentially of” or “consist of” the various components and steps.

Therefore, the various aspects of the present disclosure are well adapted to attain the ends and advantages mentioned as well as those that are inherent therein. The particular examples and configurations disclosed above are illustrative only, as the present disclosure may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular illustrative examples disclosed above may be altered, combined, or modified and all such variations are considered within the scope and spirit of the present disclosure. The aspects illustratively disclosed herein suitably may be practiced in the absence of any element that is not specifically disclosed herein and/or any optional element disclosed herein. While compositions and methods are described in terms of “comprising,” “containing,” or “including” various components or steps, the compositions and methods can also “consist essentially of” or “consist of” the various components and steps. All numbers and ranges disclosed above may vary by some amount. Whenever a numerical range with a lower limit and an upper limit is disclosed, any number and any included range falling within the range is specifically disclosed. In particular, every range of values (of the form, “from about a to about b,” or, equivalently, “from approximately a to b,” or, equivalently, “from approximately a-b”) disclosed herein is to be understood to set forth every number and range encompassed within the broader range of values. Also, the terms in the claims have their plain, ordinary meaning unless otherwise explicitly and clearly defined by the patentee. Moreover, the indefinite articles “a” or “an,” as used in the claims, are defined herein to mean one or more than one of the element that it introduces. 

1. A carbon steel composition comprising: manganese in an amount of equal to less than about 1.6% by weight or less than about 1.3% by weight of the carbon steel composition.
 2. The carbon steel composition of claim 1, wherein the manganese is in a range of about 0.6% to about 1.3% by weight of the carbon steel composition.
 3. The carbon steel composition of claim 1, wherein the manganese is in a range of about 0.6% to about 1.6% by weight of the carbon steel composition.
 4. The carbon steel composition of claim 1, further comprising one or more elements selected from the group consisting of carbon, phosphorous, sulfur, silicon, aluminum, chromium, molybdenum, niobium, titanium, nitrogen, calcium, nickel, vanadium, boron, and any combination thereof.
 5. The carbon steel composition of claim 4, wherein the carbon steel comprises nickel in an amount in the range of about 0.15% to about 1.0% by weight of the carbon steel composition.
 6. The carbon steel composition of claim 4, wherein the carbon steel comprises at least carbon in an amount of 0.01% to 0.15% by weight of the carbon steel composition.
 7. (canceled)
 8. The carbon steel composition of claim 4, wherein the carbon steel composition comprises at least niobium in an amount of about 0.02% to about 0.10% by weight of the carbon steel composition.
 9. The carbon steel composition of claim 4, wherein the carbon steel composition comprises at least vanadium in an amount of about 0.02% to about 0.10% by weight of the carbon steel composition.
 10. (canceled)
 11. The carbon steel composition of claim 4, wherein the carbon steel comprises at least chromium in an amount in the range of about 0.1% to about 0.75% by weight of the carbon steel composition.
 12. The carbon steel composition of claim 4, wherein the carbon steel comprises at least boron in an amount in the range of about 0.0005% to about 0.003% by weight of the carbon steel composition.
 13. The carbon steel composition of claim 4, wherein the carbon steel comprises at least molybdenum in an amount in the range of about 0.1% to about 0.5% by weight of the carbon steel composition.
 14. The carbon steel composition of claim 4, wherein the carbon steel comprises at least titanium in an amount in the range of about 0.005% to about 0.1% by weight of the carbon steel composition.
 15. A carbon steel composition comprising: carbon in an amount of equal to or less than about 0.025% by weight of the carbon steel composition.
 16. The carbon steel composition of claim 15, wherein the carbon is in a range of about to about 0.025% by weight of the carbon steel composition.
 17. The carbon steel composition of claim 15, further comprising one or more elements selected from the group consisting of manganese, phosphorous, sulfur, silicon, aluminum, chromium, molybdenum, niobium, titanium, nitrogen, calcium, nickel, vanadium, boron, and any combination thereof.
 18. The carbon steel composition of claim 17, wherein the carbon steel comprises at least manganese in an amount of equal to less than about 1.6% by weight or less than about 1.3% by weight of the carbon steel composition.
 19. (canceled)
 20. The carbon steel composition of claim 17, wherein the manganese is in a range of about 0.6% to about 1.6% by weight of the carbon steel composition.
 21. The carbon steel composition of claim 17, wherein the carbon steel composition comprises at least niobium in an amount of about 0.02% to about 0.10% by weight of the carbon steel composition.
 22. The carbon steel composition of claim 17, wherein the carbon steel composition comprises at least vanadium in an amount of about 0.02% to about 0.10% by weight of the carbon steel composition.
 23. The carbon steel composition of claim 17, wherein the carbon steel composition comprises at least niobium and vanadium, and the combined amount of niobium and vanadium is in an amount of about 0.02% to about 0.15% by weight of the carbon steel composition.
 24. The carbon steel composition of claim 17, wherein the carbon steel comprises at least nickel in an amount in the range of about 0.15% to about 1.0% by weight of the carbon steel composition.
 25. The carbon steel composition of claim 17, wherein the carbon steel comprises at least boron in an amount in the range of about 0.0005% to about 0.003% by weight of the carbon steel composition.
 26. (canceled)
 27. The carbon steel composition of claim 17, wherein the carbon steel comprises at least chromium in an amount in the range of about 0.1% to about 0.75% by weight of the carbon steel composition.
 28. The carbon steel composition of claim 17, wherein the carbon steel comprises at least molybdenum in an amount in the range of about 0.1% to about 0.5% by weight of the carbon steel composition.
 29. The carbon steel composition of claim 17, wherein the carbon steel comprises at least titanium in an amount in the range of about 0.005% to about 0.1% by weight of the carbon steel composition. 